This invention relates generally to digital systems. More particularly, this invention relates to a method and apparatus for calibrating a high precision delay.
Personal computers typically connect to an optical disk drive such as a CD-ROM to read data from a compact disk. On the compact disk, data is stored in the form of pits and lands patterned in a radial track. The track is formed in one spiral line extending from the inner radius of the disk to the outer edge. A pit is a location on the disk where data has been recorded by creating a depression in the surface of the disk with respect to the lands. The lands are the areas between the pits in the tangential direction. The reflectivity of the pits is less than the reflectivity of the lands. To store audio or digital information, the length of the pits and lands are controlled according to a predefined encoding format.
When reading information from the disc, light from a laser beam is directed onto the track and the light beam is reflected back to a photo-sensor such as a photo-diode. Since the pits and lands have different reflectivity, the amount of reflected light changes at the transitions between the pits and the lands. In other words, the encoded pattern of the pits and lands modulates the reflected light beam. The photo-sensor receives the reflected light beam, and outputs a modulated signal, typically referred to as an RF signal, that is proportional to the energy of the light in the reflected light beam.
In FIG. 1, the relationship of the RF signal to the pits and lands is shown. A smaller pit or land decreases both the period and the amplitude of the RF signal. The RF signal in the pits and lands has opposite polarity.
One encoding format used in optical disk systems is eight-to-fourteen modulation (EFM). EFM reduces errors by minimizing the number of zero-to-one and one-to-zero transitions. In other words, small pits and lands are avoided. A one is indicated by a change in the energy of the reflected light beam, that is, a pit edge. A zero is indicated by no change in the energy reflected beam for at least two clock periods. Applying the EFM encoding rules, a pit or land will have a length corresponding to the amount of time for at least three and up to eleven clock periods and the electronics will output a corresponding voltage as shown in FIG. 1.
The data is written on the disk via the pits and lands using EFM format. Because of the characteristics of the laser, the media and the recording speed, the EFM signal is adjusted by write strategy electronics to generate a high frequency (HF) write signal that is used to modulate the power of the laser. The write strategy electronics generates the signals to control the laser power, in addition to other control signals.
The disks can be played or written at different speeds. Therefore, the EFM data needs to be written to the disk at different speeds. As the speed increases, the period of the EFM signal and an associated reference clock signal decreases.
In FIG. 2, an ideal EFM signal corresponding to a pit or mark on the disk is shown. The ideal EFM signal is synchronized to a system clock having a clock period of T. The laser power signal needed to cause the laser to write the ideal EFM signal on the disk is also shown. The write strategy circuit generates additional control signals, EFM1, EFM2 and EFM3 that are supplied to laser interface circuitry that is used to generate the laser power signal. FIG. 2 shows the outputs of EFM1, EFM2 and EFM3 in a CD-Recordable (CD-R) drive. The EFM3 signal controls a low power pre-heat phase of the laser. The EFM2 signal controls the duration of a boost power phase of the laser. The EFM1 signal controls the overall duration of a writing phase of the laser which includes the boost power phase. In the EFM1 signal, the write strategy circuit adjusts the delay Td and Tr with respect to the rising and falling edges of the ideal EFM signal, respectively, and the system clock. In the EFM2 signal, the write strategy circuit adjusts the duration of the EFM2 pulse Tw with respect to the rising edge of the EFM1 signal. In the EFM3 signal, the write strategy circuit adjusts a delay Th with respect to the ideal EFM signal and the system clock. To write data accurately, each timing parameter, Td, Tr, Tw and Th must be adjusted with very high precision, such as {fraction (1/32)} T. In one embodiment, {fraction (1/32)} T is equivalent to 0.9 nanoseconds.
One method to delay a signal uses combinational logic to adjust the timing parameters such as the delay Td. One problem with this method is that it is difficult to precisely calculate the exact amount of delay during the design stage. The amount of delay also changes because of process, temperature and power supply variations.
In view of the foregoing, it would be highly desirable to provide a method and apparatus that precisely delays an input signal for a desired amount of time. The method and apparatus should also provide a stable and precise amount of delay that accommodates process, temperature and power supply variations. Such a circuit would reduce timing problems in digital environments requiring high precision delays, such as in CD-ROM drives, CD-R and CD-Rewritable (CD-RW) drives.
A circuit provides a predetermined amount of high precision delay under temperature, power supply and process variations. An oscillator generates oscillator pulses, each with an oscillator period. A counter, in response to an enable signal, counts the oscillator pulses and outputs a count signal. The enable signal is a reference clock signal having a reference-clock period that is greater than the oscillator period. A delay generator delays an input signal to provide a sequence of incrementally delayed delay-signals. A multiplexor, in response to the count signal, selects one of the delayed signals.
In another aspect of the invention, a disc controller uses the circuit to provide a high precision delay to a write data signal to control laser power. Another aspect of the invention provides a method of providing the high precision delay.
In this way, by allowing the count signal to increase or decrease with the speed of the oscillator, the invention provides substantially the same amount of delay to the input signal under temperature, power supply or process variations.